Given the importance of the liver for homeostasis, tests of liver function are a mainstay of clinical diagnosis. Such tests have several goals: (1) to assess whether hepatocytes have been injured or are dysfunctional, (2) to determine whether bile excretion has been interrupted, and (3) to evaluate whether cholangiocytes have been injured or are dysfunctional. Liver function tests are also used to monitor responses to therapy or rejection reactions after liver trans •Fig. 32.15 Ammoniahomeostasisinhealth.(RedrawnfromBarrettKE.Gastrointestinal Physiology. NewYork:McGraw-Hill;2006.) by hepatocytes, where it then enters the urea cycle to be converted to urea (see
Fig. 32.14 ) and is subsequently transported back into the systemic circulation. Urea is a small neutral molecule that is readily filtered at the glomerulus, and it is reabsorbed by the kidney tubules such that approximately 50% of the filtered urea is excreted in urine (see ). Urea that enters the colon is either excreted or metabolized to ammonia via colonic bacteria, with the resulting ammonia being reabsorbed or excreted.
plantation. However, not all such tests measure function directly. Nevertheless, liver function tests are discussed briefly because of their link to hepatic physiology.
Tests for hepatocyte injury rely on markers that are specific for this cell type. When hepatocytes are killed by necrotic responses to inflammation or infection, for example, they release enzymes that include alanine aminotransferase (ALT) and aspartate aminotransferase (AST). These enzymes, which are essential to interconvert amino acids, are easily measured in serum and indicate hepatocyte injury, although AST may also be released after injury to other tissues, including the heart. Two other tests are markers of injury to the biliary system. Alkaline phosphatase is expressed in the canalicular membrane, and elevations of this enzyme in plasma suggest localized obstruction to bile flow. Similarly, increased levels of GGT are seen when there is damage to cholangiocytes.
Measurement of bilirubin in the circulation or in urine also provides insight into liver function. In addition, measurement of any of the other characteristic secreted products of the liver can be used to diagnose liver disease. Clinically the most common tests are measurements of serum albumin and a blood clotting parameter, the prothrombin time. If results of these tests are abnormal, when considered together with other aspects of the clinical picture, a diagnosis of liver
CHAPTER 32 Transport and Metabolic Functions of the Liver disease may be established. Blood glucose and ammonia levels are frequently monitored in patients with chronic liver disease. Finally, imaging tests and histological examination of biopsy specimens of liver parenchyma, usually obtained percutaneously, are also important in evaluating and monitoring patients with suspected or proven liver disease.
1.
Vital functions of the liver include carbohydrate, lipid, and protein metabolism and synthesis; detoxification of unwanted substances; and excretion of circulating substances that are lipid soluble and carried in the bloodstream bound to albumin. The liver also synthesizes the majority of plasma proteins, including albumin.
2.
Liver function depends on its unique anatomy, its constituent cell types (especially hepatocytes), and the unusual arrangement of its blood supply.
3.
Substances are excreted from the liver in bile. Bile flow is driven by the presence of bile acids, which are
Feranchak AP. Bile secretion and cholestasis. In: Podolsky DK, et al., eds. Yamada’s Textbook of Gastroenterology. 6th ed. Hoboken, NJ: Wiley Blackwell; 2015.